There is an electronically controlled engine whose target rotational speed can be directly specified by an operator. Such an engine includes an operation lever, an AD conversion device, and an engine rotational speed control device for setting the target rotational speed. The AD conversion device generates a command value of the target rotational speed for each step per unit time by digital converting an analog value that is input by the operation lever. The engine rotational speed control device controls the amount of fuel supply based on the generated command value.
The error in AD conversion, a signal noise, a slight vibration in the operation lever, and the like cause an error to occur in the command value of the target rotational speed. As a result, the command value of the target rotational speed varies in the range of several LSBs (Least Significant Bit) with respect to the analog value of the target rotational speed input by the operator. A slight variation in the target rotational speed may cause hunting of the engine. Especially, such hunting occurs when the target rotational speed corresponds to the switching rotational speed of fuel injection patterns. In this case, the target rotational speed varies across the switching rotational speed, and the fuel injection pattern is frequently switched. The operator gets a strange feeling regarding the operation state of the engine because the operator hears frequent variation in the engine sound even though the operator is not operating the operation lever.
Patent Document 1 discloses an example of a technique for correcting a control signal. In paragraph 0025 of Patent Document 1, an averaging process on a calculation value of the accelerator opening is described. According to paragraph 0027 of Patent Document 1, a radical increase in the pressure inside an intake passage at the time of deceleration when a turbo charger 31 is operating can be prevented by this averaging process.